1. Technical Field of the Invention
The present invention relates to an axial flow compressor which can reduce a tip clearance loss so as to inhibit a stall.
2. Description of the Related Art
FIG. 1 is a schematic view of a structure of a turbojet engine. The turbojet engine is provided with an air intake port 1, a compressor 2, a combustor 3, a gas turbine 4, an after burner 5, a jet nozzle 6 and the like. In the turbojet engine mentioned above, the structure is made such that an air is introduced from the air intake port 1, the air is compressed by the compressor 2, a fuel is burnt within the combustor 3 so as to generate a high temperature combustion gas, the gas turbine 4 is driven by the generated combustion gas, the compressor 2 is driven by the gas turbine 4, the fuel is reheated by an exhaust gas getting out of the turbine by the after burner 5, and the high temperature combustion exhaust gas is expanded by the jet nozzle 6 so as to be injected to a rear side, whereby a thrust is generated. The basic structure is approximately the same in the other jet engines than the turbojet engine.
In the jet engine mentioned above, the other gas turbine, or a compressor simple substance, a clearance between a leading end (a tip) of a rotor blade constituting the compressor and a casing inner surface is generally called as a tip clearance. The tip clearance is preferably set to be always smaller in view of improving a compression efficiency, however, since the tip clearance is affected in an actual operation by (1) an elongation of the rotor blade due to a centrifugal force, (2) a thermal expansion of the rotor blade, (3) a thermal expansion of the casing and the like, the tip clearance is variously fluctuated in correspondence to an operational state.
FIG. 2 is a view schematically showing a change of the tip clearance at a time of starting. In a stopped state of the compressor, all of the rotor blade, and the rotor, the casing and the like to which the rotor blade is attached are at a room temperature (for example, 20 to 30° C.), and the tip clearance comes to the maximum (a point A). When the compressor is started and the rotor blade starts rotating, the rotor blade is elongated due to the centrifugal force and the tip clearance becomes small (a point B). Next, the temperature of the compressed air is increased on the basis of an adiabatic compression of the air, whereby the rotor blade having a small heat capacity is first thermally expanded and the tip clearance becomes smaller to the minimum (a point C). Next, the casing and the rotor having a large heat capacity are thermally expanded, the tip clearance is gradually increased, and comes to an approximately fixed design range (a point D) in a steady state.
In order to keep the tip clearance mentioned above in an optimum state, for example, Japanese Unexamined Patent Publication No. 06-317184 has been already filed.
A “tip clearance control apparatus” described in Japanese Unexamined Patent Publication No. 06-317184 is provided, as shown in FIG. 3, with a temperature detector 14a attached to a casing 13, a cooling tube 15a in which one end is connected to a compressor portion 16, the other end is open to an ambient air and an intermediate portion is attached to the casing 13 so as to be positioned near the temperature detector 14a, a flow rate adjusting valve 17a provided in the cooling tube 15a, a heat deformation data storage device 18 storing a relation between a tip clearance C in a leading end portion of a rotor blade 12 with respect to an inner side portion of the casing 13 and a temperature of a portion to which the temperature detector 14a is attached as a heat deformation data signal 19, and a heat deformation controller 20 outputting a flow rate adjusting signal 22 to the flow rate adjusting valve 17a on the basis of a temperature detection signal 21a output from the temperature detector 14a and the heat deformation data signal 19, and is structured such as to locally cool the casing 13 by circulating an air from the compressor portion 16 to the cooling tube 15a. 
In this case, a flow circulating through the blade row is disclosed, for example, in documents 1, 2 and 3.
[Document 1]
Gas Turbine Engine, page 41, written by Yoshimichi Tanida and Toshio Nagashima, and issued by ASAKURA BOOK SHOP
[Document 2]
Horlock, Axial Flow Compressor, Butterworths Publications Limited
[Document 3]
AERODYNAMIC DESIGN OF AXIAL-FLOW COMPRESSORS, NASA SP-36
As shown in FIG. 2, the tip clearance is variously changed from the start time to the steady state. Accordingly, the “tip clearance control apparatus” is effective only in the case that the tip clearance is large.
Accordingly, the minimum tip clearance is set such that the leading end (the tip) of the rotor blade is not in contact with an inner surface of the casing at the point C. Therefore, in the conventional compressor, it is practically impossible to always keep the tip clearance minimum.
As mentioned above, the tip clearance is fluctuated on the basis of the operational state (a rotating speed, a compression ratio, an outside air temperature, or the like) of the compressor. In some operational state, not only the tip clearance becomes too large, and the compression efficiency is lowered, but also a flow (called as a tip clearance flow) is generated from a positive pressure surface to a negative pressure surface in the tip clearance portion between a tip end of the rotor blade and an outside passage. The tip clearance flow corresponds to a factor causing a stall which inhibits a stable operation at a time when the blade is made so as to use in high load, and a main factor of a surging phenomenon corresponding to a working limit of the compressor.